Process for producing tungsten oxide and tungsten mixed oxides

ABSTRACT

Process for producing a tungsten oxide powder or a tungsten mixed oxide powder of general formula M x WO 3 , wherein M=Na, K, Rb, Li and/or Cs, 0.1≤x≤0.5, comprising the consecutive steps of:
         a) providing a solution comprising respectively at least one tungsten compound and optionally at least one M-comprising compound in a concentration corresponding to the stoichiometry M x WO 3 ,   b) atomizing the solution, thus forming an aerosol, into a reaction space,   c) reacting the aerosol in the reaction space with a hydrogen/oxygen flame for which the expression 1&lt;O 2,primary /0.5H 2 ≤3 applies,
           wherein the reaction space is configured such that it comprises two reaction zones with two different velocities of the reaction mixture v 1  and v 2  where v 2 =0.3-0.8 v 1  and 0.5≤v 1 ≤10 Nm/s,   
           d) separating the solid from vaporous or gaseous substances and   e) passing a reducing gas stream over the separated solid at a temperature of 450-700° C.       

     Process for producing a tungsten oxide powder or a tungsten mixed oxide powder of general formula M x WO 3 , wherein M=Na, K, Rb, Li and/or Cs, 0.1≤x≤0.5, comprising the consecutive steps of:
         a) providing a solution comprising respectively at least one tungsten compound and optionally at least one M-comprising compound in a concentration corresponding to the stoichiometry M x WO 3 ,   b) atomizing the solution, thus forming an aerosol, into a reaction space,   c) reacting the aerosol in the reaction space with a hydrogen/oxygen flame having a lambda value&lt;1, wherein: lambda=total oxygen/0.5×hydrogen,   d) separating the solid from vaporous or gaseous substances.

The invention relates to a process for producing tungsten oxide powders by means of flame spray pyrolysis.

Tungsten oxides are known infrared-absorbing substances which can also show good electrical conductivity. Such a mixed oxide is generally obtained by providing a solution comprising a tungsten compound and optionally a compound of a mixed oxide component, subsequently removing the solvent and treating the remaining solid at temperatures of about 500° C. in a reducing atmosphere. Mention may be made for example of EP1801815 which describes the production of a tungsten mixed oxide of formula M_(x)W_(y)O_(z) where M=alkali metal, 0.001≤x/y≤1.1 and 2.2≤z/y≤3.0. Powders produced in this way often show poor dispersibility, for example in a coating. Accordingly, costly and complex grinding and dispersing steps are often necessary.

In US 2010/102700, tungsten mixed oxides are produced by means of flame spray pyrolysis. A solution comprising a tungsten compound and a compound of the mixed oxide component is introduced into a flame and oxidized there. However, only materials having a BET surface area of about 60-80 m²/g are disclosed. To achieve sufficient crystallinity, the material from the flame spray pyrolysis is subsequently subjected to thermal treatment which reduces the BET surface area to values of about 20 m²/g. This has a deleterious effect on dispersibility.

A process allowing production of tungsten oxides having high crystallinity and good dispersibility would be desirable.

The invention provides a single-stage process and a two-stage process for producing tungsten oxide or a tungsten mixed oxide. Both processes comprise a flame spray pyrolysis.

The two-stage process for producing a tungsten oxide powder or a tungsten mixed oxide powder of general formula M_(x)WO₃, wherein M=Na, K, Rb, Li and Cs, 0.1≤x≤0.5, preferably x=0.33, comprises the consecutive steps of

-   -   a) providing a solution comprising respectively one or more         tungsten- and optionally M-comprising compounds in a         concentration corresponding to the stoichiometry M_(x)WO₃,     -   b) atomizing the solution or the solutions, thus forming an         aerosol, into a reaction space,     -   c) reacting the aerosol in the reaction space with a         hydrogen/oxygen flame for which the expression         1<O_(2,primary)/0.5H₂≤3 applies,         -   wherein the reaction space is configured such that it             comprises two reaction zones with two different velocities             of the reaction mixture v₁ and v₂ where v₂=0.3-0.8 v₁,             preferably v₂=0.4-0.7 and 0.5≤v₁≤10 Nm/s, preferably 1≤v₁≤5             Nm/s,     -   d) separating the solid from vaporous or gaseous substances and     -   e) passing a reducing gas stream over the separated solid at a         temperature of 450-700° C.

The BET surface area of the tungsten oxide powder produced by the process according to the invention is 1-10 m²/g.

The process according to the invention is particularly suitable for producing WO₃, Li_(0.33)WO₃, Na_(0.33)WO₃, K_(0.33)WO₃, Rb_(0.33)WO₃, Cs_(0.33)WO₃, Cs_(0.20)WO₃ and Cs_(0.25)WO₃. It is likewise possible to produce tungsten mixed oxide powders of general formula W_(x1)M_(x2)WO₃, where x1+x2=x, which in addition to tungsten comprise two metals selected from the group Na, K, Rb, Li and Cs, for example Na_(0.1)K_(0.1)WO₃.

The first reaction zone begins at the point of introduction of the aerosol into the reaction space. The second reaction zone is immediately downstream of the first. v₁ is the average velocity in the first reaction zone, v₂ the average velocity in the second reaction zone. v₁ is greater than v₂. This can be achieved for example by a smaller cross section in the first reaction zone. v₁ and v₂ are calculated based on the gas volumes of the unconverted starting materials, for example nitrogen or excess oxygen, and of the products, substantially water vapour. The velocity figures are normalized velocities. They are found by dividing the volume flow rate having the unit Nm³/h by the cross-sectional area.

The index “O₂, primary” refers to the oxygen in the air or optionally in the oxygen-enriched air which forms the flame. In addition it may be useful to additionally introduce, separately from this air, air referred to here as secondary air directly into the reaction space. The index “O₂, secondary” then refers to the oxygen in the secondary air. The secondary air is preferably directed such that it enters the reaction space only in reaction zone 2.

Finally, the atomization of the solution may also be effected by means of air. The index “O₂, atomization” then refers to the oxygen in the atomization air. Finally, the index “O₂, ttl” refers to the total oxygen. The total oxygen is preferably chosen such that

1.5≤O _(2,ttl)/0.5H ₂≤5.

The ratio O_(2,ttl)/0.5H₂ or O_(2,primary)/0.5H₂ stems from the reaction of 2 mol of hydrogen with 1 mol of oxygen as per 2H₂+O₂→2H₂O.

The claimed features in terms of O_(2,primary)/0.5H₂ and v₂=0.4-0.8 v₁ result in low temperatures of about 500-750° C. in the flame spray pyrolysis and in a material having a surprisingly low BET surface area of 1-20 m²/g coupled with good crystallinity.

The BET surface area of the reduced material is smaller than that of the material from the flame spray pyrolysis. The ratio BET_(reduced)/BET_(FSP) is normally 0.60-0.95.

The hydrogen/oxygen flame which burns into the reaction space is formed by igniting an oxygen-comprising gas and a combustion gas which upon reaction with oxygen forms water.

In a particular embodiment the average residence times of the reaction mixture in reaction zone 1 and reaction zone 2 are chosen such that t₂>0.5 t₁, preferably 0.7 t₁≤t₂≤0.9 t₁, wherein t₁ is the average residence time of the reaction mixture in reaction zone 1 and t₂ is the average residence time of the reaction mixture in reaction zone 2.

The average residence time t₁ is preferably 0.2-1 s, particularly preferably 0.3-0.7 s. The average residence time t₂ is preferably 0.1-0.8 s, particularly preferably 0.2-0.5 s.

Contemplated reducing gas streams are hydrogen, hydrogen/nitrogen mixtures or hydrogen/noble gas mixtures.

In addition to the two-stage process comprising flame spray pyrolysis and a subsequent reduction, a single-stage process where the reduction step may be eschewed also forms part of the subject matter of the invention.

The invention provides a single-stage process for producing a tungsten oxide powder or a tungsten mixed oxide powder of general formula M_(x)WO₃, wherein M=Na, K, Rb, Li and/or Cs, 0.1≤x≤0.5, comprising the consecutive steps of

-   -   a) providing a solution comprising respectively at least one         tungsten compound and optionally at least one M-comprising         compound in a concentration corresponding to the stoichiometry         M_(x)WO₃,     -   b) atomizing the solution, thus forming an aerosol, into a         reaction space,     -   c) reacting the aerosol in the reaction space with a         hydrogen/oxygen flame having a lambda value<1, wherein:         lambda=total oxygen/0.5×hydrogen, and     -   d) separating the solid from vaporous or gaseous substances.

It is an essential feature of the process that lambda<1. It means that hydrogen and oxygen are to be chosen such that there is a stoichiometric excess of hydrogen in terms of the equation H₂+0.5 O₂→H₂O. It is preferable when 0.6≤lambda<1, particularly preferable when 0.7≤lambda≤0.95 and very particularly preferable when 0.8≤lambda≤0.9.

The average residence time in the reaction space for the single-stage process is preferably 1-5 s.

In the process according to the invention fine droplets of the solution are a constituent of the aerosol. The fine droplets preferably have an average droplet size of less than 120 μm, particularly preferably of 30-100 μm. The droplets are typically produced using single- or multi-component nozzles.

The solution employed shall have as high a concentration as possible. The intention is to achieve an optimum balance between production output and the properties of the powder. For the processes according to the invention these requirements are best fulfilled in a range of 5-60 wt %, particularly preferably 25-55 wt %, very particularly preferably 30-50 wt %, in each case based on the sum of W and M and based on the metal.

The best results in terms of homogeneity of the powder are obtained when the tungsten and alkali metal compounds are present in a solution. The solution may be heated to achieve solubility and to attain a suitable viscosity for atomization of the solution. All soluble metal compounds convertible into the oxides under the reaction conditions may in principle be employed. These may be inorganic metal compounds, such as nitrates, chlorides, bromides, or organic metal compounds, such as alkoxides or carboxylates. Nitrates may be particularly advantageously employed.

Organic solvents that may be employed with preference are alcohols such as methanol, ethanol, n-propanol, isopropanol, n-butanol or tert-butanol, diols such as ethanediol, pentanediol, 2-methylpentane-2,4-diol, acetic acid, propionic acid, butanoic acid, hexanoic acid, oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, octanoic acid, 2-ethylhexanoic acid, valeric acid, capric acid or lauric acid. Benzene, toluene, naphtha and/or spirits may also be employed. Preference is given to using an aqueous solvent or water.

EXAMPLES

Examples 1-5 show the production of alkali metal-tungsten mixed oxide powders by the two-stage process according to the invention. Example 6 shows a comparative example of a two-stage process. Examples 7-11 show the production of potassium-tungsten mixed oxide powders by the single-stage process according to the invention.

Example 1

A solution of 2165 g of ammonium metatungstate, 335 g of caesium nitrate and 12 020 g of water is produced. The total concentration of W and Cs, as metal in each case, is 12.7 wt %.

Reaction zone 1: Jetting 2500 g/h of this solution with 5 Nm³/h of air as jetting gas by means of a two-material nozzle at room temperature (23° C.) affords an aerosol. This is brought to reaction with 8 Nm³/h (0.357 kmol/h) of hydrogen and 30 Nm³/h of air (0.281 kmol 02/h). The temperature 50 cm below the burner mouth is 527° C.

The residence time in reaction zone 1 is 0.48 seconds at a gas velocity of 2.89 Nm/s.

Reaction zone 2: 15 Nm³/h of secondary air (0.141 kmol 02/h) are additionally introduced into the reactor outside reaction zone 1.

The residence time in reaction zone 2 is 0.36 seconds at a gas velocity of 1.65 Nm/s. Subsequently, the reaction mixture is cooled and the obtained solid separated from the gaseous materials on a filter.

The solid has a BET surface area of 7.2 m²/g.

The solid from the FSP is heated under a nitrogen atmosphere at a heating rate of 8.0° C./min to an end temperature of 500° C. and there treated in a forming gas atmosphere (70/30 vol % N₂/H₂,volume flow 100 NI/h) over a period of 2 hours at a temperature of 500° C.

The obtained reduced solid has a BET surface area of 5.4 m²/g. A dispersion of the solid, 18 wt % in 1-methoxy-2-propanol, is deep blue in colour.

Examples 2 to 6 are performed analogously. Example 6 is a comparative example where the average velocities in both reaction zones are identical. Starting materials and reaction conditions may be found in table 1. Table 2 shows calculated values.

The two-stage process according to the invention is directed at obtaining in a flame pyrolytic process a material having a low BET surface area coupled with a rather high crystallinity. In the subsequent reduction step the BET surface area is reduced only to a small extent while the reduction proceeds rapidly under moderate conditions without appreciable sintering. This material is correspondingly readily dispersible.

In comparative example 6 a material having an increased BET surface area is obtained from the FSP. This material is more difficult to reduce. The reduced material itself is more difficult to disperse.

Example 7

A solution of 4316 g of ammonium metatungstate, 502 g of potassium acetate, 64 g of glacial acetic acid and 4831 g of water is produced. The total concentration of W and K, in each case as metal, is 34.2 wt %.

Jetting 9.0 kg/h of this solution with 9.2 Nm³/h of air as jetting gas by means of a two-material nozzle at room temperature (23° C.) affords an aerosol. This is brought to reaction with 10 Nm³/h of hydrogen and 13.5 Nm³/h of air. The temperature 50 cm below the burner mouth is 464° C. Lambda is 0.87. The residence time is 1.9 seconds.

The solid has a BET surface area of 2.4 m²/g. A dispersion of the solid, 18 wt % in 1-methoxy-2-propanol, is deep blue in colour. X-ray structural analysis shows a hexagonal potassium-tungsten mixed oxide.

Examples 8 to 11 are performed analogously with the same solution. The reaction conditions may be found in table 3.

Examples 7 to 11 show that reduced powders are producible by means of a single-stage process, namely a flame-spray pyrolysis.

TABLE 1 Two-stage process - starting materials - BET surface area of the powders Example 1 2 3 4 5 6 (comp.) solution ammonium g/h 216.5 241.5 143.8 145.3 144.9 216.5 metatungstate caesium nitrate g/h 33.5 — — — — 33.5 potassium nitrate g/h — 8.5 — — 3.2 — sodium nitrate g/h — — 6.2 47.4 1.9 — concentration^(a)) wt % 12.7 13.0 13.0 13.0 13.0 12.7 gases atomizer air Nm³/h 5 5 5 5 5 5 hydrogen Nm³/h 8 6 6 6 6 8 primary air Nm³/h 30 30 30 30 30 30 secondary air Nm³/h 15 15 15 15 15 53 temperature^(b)) ° C. 527 531 525 528 524 524 red. atmosphere vol % 70:30 N₂/H₂ ^(c)) T_(reduction)/t_(reduction) ° C./h 500/2 BET_(FSP) m²/g 7.2 3.9 2.0 2.1 2.1 16.2 BET_(reduced) m²/g 5.4 3.5 1.8 1.8 2.0 5.8 BET_(reduced)/BET_(FSP) 0.75 0.90 0.90 0.86 0.95 0.36 ^(a))based on metals; ^(b))reaction zone 1; 50 cm below reactor inlet; ^(c))volume flow 100 Nl/h

TABLE 2 Two-stage process - calculated values 6 example 1 2 3 4 5 (comp.) H₂ kmol/h 0.357 0.268 0.268 0.268 0.268 0.357 O_(2, primary) kmol/h 0.281 0.281 0.281 0.281 0.281 0.281 O_(2, secondary) kmol/h 0.141 0.141 0.141 0.141 0.141 0.496 O_(2, atomiz.) kmol/h 0.047 0.047 0.047 0.047 0.047 0.047 O_(2, ttl) kmol/h 0.469 0.469 0.469 0.469 0.469 0.824 O_(2, primary)/0.5H₂ 1.57 2.10 2.10 2.10 2.10 1.57 O_(2, primary+atomiz.)/ 1.84 2.45 2.45 2.45 2.45 1.84 0.5H₂ O_(2, ttl)/0.5H₂ 2.63 3.50 3.50 3.50 3.50 4.62 v₁ ^(a)) Nm/s 2.89 2.77 2.74 2.75 2.74 2.89 v₂ ^(a)) Nm/s 1.65 1.61 1.45 1.59 1.57 2.89 t1^(b)) s 0.48 0.51 0.51 0.51 0.51 0.48 t2^(b)) s 0.36 0.37 0.41 0.38 0.38 0.21 v₂/v₁ 0.57 0.58 0.53 0.58 0.57 1.00 t₂/t₁ 0.75 0.74 0.81 0.74 0.75 0.43 ^(a))v₁, v₂ = average gas velocity reaction zone 1, 2; ^(b))average residence time reaction zone 1, 2;

TABLE 3 single-stage process - starting materials - BET surface area of the powders example 7 8 9 10 11 solution ammonium g/h 4316 metatungstate potassium g/h 502 acetate solution^(a)) wt % 34.2 34.2 34.2 34.2 42.8 throughput kg/h 9.0 6.0 6.0 9.0 12.0 gases air Nm³/h 13.5 13.5 13.5 13.5 13.5 atomizer air Nm³/h 9.2 9.2 9.4 9.5 9.5 total oxygen Nm³/h 4.77 4.77 4.81 4.83 4.83 hydrogen Nm³/h 10.0 10.0 12.0 12.0 12.0 lambda^(b)) 0.87 0.87 0.80 0.81 0.81 temperature^(c)) ° C. 464 449 587 566 590 BET surface m²/g 2.4 2.0 1.3 1.7 1.5 area ^(a))based on metals; ^(b))lambda = total oxygen/0.5 × hydrogen; ^(c))reaction zone 1; 50 cm below reactor inlet 

1-11. (canceled)
 12. A process for producing a tungsten oxide powder or a tungsten mixed oxide powder of general formula M_(x)WO₃, wherein M=Na, K, Rb, Li and/or Cs, and 0.1≤x≤0.5, comprising the consecutive steps of: a) providing a solution comprising respectively at least one tungsten compound and optionally at least one M-comprising compound in a concentration corresponding to the stoichiometry M_(x)WO₃; b) atomizing the solution, thus forming an aerosol, into a reaction space; c) reacting the aerosol in the reaction space with a hydrogen/oxygen flame for which the expression 1<O_(2,primary)/0.5H₂≤3 applies, and wherein the reaction space is configured such that it comprises two reaction zones with two different velocities of the reaction mixture v₁ and v₂ where v₂=0.3−0.8 v₁ and 0.5≤v₁≤10 Nm/s; d) separating the solid from vaporous or gaseous substances; and e) passing a reducing gas stream over the separated solid at a temperature of 450-700° C.
 13. The process of claim 12, wherein total oxygen is such that 1.5≤O_(2,ttl)/0.5H₂≤5.
 14. The process of claim 12, wherein t₂>0.5 t₁, and wherein t₁ is the average residence time of the reaction mixture in reaction zone 1 and t₂ is the average residence time of the reaction mixture in reaction zone
 2. 15. The process of claim 12, wherein the thermal treatment is performed over a period of 1-10 hours.
 16. The process of claim 12, wherein hydrogen, hydrogen/nitrogen mixtures or hydrogen/noble gas mixtures are employed as the reducing gas stream.
 17. The process of claim 12, wherein the average residence time in the reaction space is 1-5 s.
 18. The process of claim 17, wherein atomization is effected by means of a single- or multimaterial nozzle and the mean droplet diameter of the aerosol is less than 120 μm.
 19. The process of claim 18, wherein the concentration of metals in the solution is 5-60 wt %.
 20. The process of claim 12, wherein atomization is effected by means of a single- or multimaterial nozzle and the mean droplet diameter of the aerosol is less than 120 μm.
 21. The process of claim 12, wherein the solution comprises inorganic metal compounds.
 22. The process of claim 12, wherein the solution is an aqueous solution.
 23. The process of claim 12, wherein the concentration of metals in the solution is 5-60 wt %.
 24. A process for producing a tungsten oxide powder or a tungsten mixed oxide powder of general formula M_(x)WO₃, wherein M=Na, K, Rb, Li and/or Cs, and X is 0.1≤x≤0.5, comprising the consecutive steps of: a) providing a solution comprising respectively at least one tungsten compound and optionally at least one M-comprising compound in a concentration corresponding to the stoichiometry M_(x)WO₃; b) atomizing the solution, thus forming an aerosol, into a reaction space; c) reacting the aerosol in the reaction space with a hydrogen/oxygen flame having a lambda value<1, wherein: lambda=total oxygen/0.5×hydrogen; d) separating the solid from vaporous or gaseous substances.
 25. The process of claim 24, wherein atomization is effected by means of a single- or multimaterial nozzle and the mean droplet diameter of the aerosol is less than 120 μm.
 26. The process of claim 24, wherein the average residence time in the reaction space is 1-5 s.
 27. The process of claim 26, wherein atomization is effected by means of a single- or multimaterial nozzle and the mean droplet diameter of the aerosol is less than 120 μm.
 28. The process of claim 27, wherein the concentration of metals in the solution is 5-60 wt %.
 29. The process of claim 24, wherein the solution comprises inorganic metal compounds.
 30. The process of claim 24, wherein the solution is an aqueous solution.
 31. The process of claim 24, wherein the concentration of metals in the solution is 5-60 wt %. 